a) Field of the Invention
The invention relates to a pump unit, in particular for heating and cooling installations, the speed of which can be altered by changing the operating frequency of a frequency converter feeding the electric motor, and having a device which protects the unit from thermal overload, the pump unit and frequency converter forming a structural unit.
b) Description of the Prior Art
Pump units of this type are used as circulating pumps, for example in heating installations. A pump unit of this type is disclosed, for example in German Offenlegungsschrift 36 42 729. A pump unit of this type is usually provided with a protective motor switch which switches the motor off in the case of thermal overload. A thermostat arranged in the region of the motor winding and which drives a relay arranged in the terminal box is usually provided for this purpose.
Switching off the pump unit when there is thermal overload usually results in complete breakdown of the plant, in particular when used in heating and cooling installations, unless as is conventional for example in the case of heating installation in hospitals, a second pump unit is provided which is switched on automatically if there is a problem with the first one. A device of this type is disclosed, for example in German Offenlegungsschrift 37 06 034. In devices of this type the protective motor switch does therefore not suffice as such, it is additionally necessary to install a second pump unit operating completely independently of the first, to ensure the operational safety of the plant even if a pump unit fails. The constructional and hence also financial burden for devices of this type is correspondingly high. However, even the incorporation of the protective motor switch conventionally provided in or on a pump unit of this type represents a not inconsiderable cost factor in relation to the total cost. Particularly in the case of pump units described in the introduction which are equipped with a frequency converter to adjust the speed, it is already difficult to arrange the relay of the protective motor switch on the pump unit for reasons of space, since the terminal box otherwise conventionally provided for this purpose is required as installation space for the frequency converter or parts of it.
Starting from the state of the art mentioned in the introduction the main object of the present invention is to design a generic pump unit in a simple and cost effective manner, so that on the one hand a protective motor switch of the type described above can be dispensed with for reasons relating to the constructional size and the constructional costs, and on the other hand a certain emergency facility is ensured even when the pump unit is thermally overloaded, so that failure of a plant supplied by the pump unit is avoided.